Myelin, a multi-layered membranous sheath enwrapping individual axons, is required for both the fast conduction of nerve impulses and for axonal function and integrity (1,2). Myelin is synthesized by oligodendroglia in the central nervous system (CNS) a process occurring shortly after the birth. Defects with myelin cause severe diseases in human; multiple sclerosis (MS) is one of such diseases, characterized by severe loss of myelin in the CNS. Although the disease was first described more than a century ago, both the etiology and cure remain largely unknown and uninvestigated (3). Only recently, a population of oligodendroglial precursor cells was first observed in MS lesions (4). Unravaling mechanisms that enable those endogenous precursor cells to be differentiated and/or strengthened so as to regenerate the lost myelin would prove to be an efficient therapeutic target. Therefore, comprehensive studies for elucidating the mechanism of myelinogenesis are of great importance in the treatment and cure of demyelinating diseases.
Myelin is composed of a limited number of myelin proteins. Myelin basic protein (MBP) comprises 30-40% of all myelin proteins within the CNS. MBP is indispensable in myelinogenesis, playing a crucial role in the compaction of the myelin sheath (5). MBP as regulates the expression of other myelin proteins such as myelin-associated glycoprotein (MAG), indicating that MBP is extremely important for myelinogenesis (6). The spontaneous Shiverer mouse, which has a natural knock-out of the MBP gene (7), exhibits severe hypomyelination of CNS axons, leading to premature death within 3 months. Specific isoforms of MBP containing exon 2 of the MBP gene play a regulatory role in myelinogenesis (8), suggesting that the preliminary events of myelinogenesis require MBP.
In the process of myelinogenesis, Fyn tyrosine kinase (Fyn) functions as an essential signaling molecule within oligodendroglia (9), stimulating the expression of MBP (10). Mice deficient in Fyn suffer severe decreases in MBP production, resulting in severe dysmyelination (9-11). Fyn, a non-receptor type tyrosine kinase, requires coupling to an adapter molecule in order to receive extracellular signals. Although myelin-associated glycoprotein (MAG) has been proposed as a candidate molecule responsible for the initial triggering of Fyn signals in myelination (9-13), MAG-deficient mice demonstrated only subtle myelin abnormalities (14,15). No significant differences in MBP expression were observed in MAG-deficient mice (14-16), casting doubt on the validity of MAG as the upstream signaling molecule which links extracellular signals to Fyn. Several researchers postulated that compensatory molecules may function in MAG-deficient mice; such molecules, however, have yet to be identified (17,18).
It was suggested that MAG is not the trigger of myelinogenesis but that hitherto unknown molecules are responsible for triggering (6). In addition, in vitro studies have revealed that the activation of Fyn during the morphological differentiation of oligodendroglia occurs prior to the first expression of MAG (19). It is MBP that regulates the expression of MAG in vivo (6).
It is an object of the present invention to identify the trigger that functionally couples to Fyn to cause the initial expression of MBP, thereby obtaining therapeutic strategies for diseases that result from defects with myelinogenesis. This molecule, i.e. the true trigger of myelinogenesis, must be expressed prior to the expression of MAG (i.e. early in the second week after birth; 6,20). Elucidation of this signal cascade will allow close understanding of the mechanism underlying myelinogenesis.